There have been many utility knives provided in the prior art. Such knives usually comprise or include a two part housing secured together by a screw and having a slidable blade therein with an external thumb piece connected to a carriage for the blade to slide it in and out of the casing at an end thereof for the purpose of opening cartons and the like. However, the knives commonally utilized are originally almost as sharp as a razor, and such knives are found to be dangerous and may accidentally cut the person using the knife because of the fact that once the blade is projected to the degree desired, it stays in this exposed position until it is deliberately manually retracted. It is the object of the present invention to provide a safety utility knife in which the blade is normally housed completely and is only projected and held in projected form by the thumb of the user when in use, there being a spring to retract the blade upon release of the external thumb piece.